This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 199 63 373.8 filed in Germany on Dec. 28, 1999; the entire content of which is hereby incorporated by reference.
The invention relates to an arrangement for cooling a flow-passage wall surrounding a flow passage.
The increase in power of gas-turbine plants and the desire for higher efficiencies is closely linked with the demand for higher process temperatures, which occur due to the combustion of a fuel/air mixture inside the combustion chamber. However, the desire for higher process temperatures, which can be entirely fulfilled with modern combustion techniques, is in turn subject to material limits on account of the plant components which can only be thermally loaded to a limited degree and are directly exposed to the hot gases produced by the combustion inside the combustion chamber. In order to increase the process temperatures on the one hand, and thus increase the thermodynamic efficiency of a gas-turbine plant, but so that they nonetheless lie below the thermal melting-point level of the respective materials from which the individual gas-turbine plant components, such as, for example, turbine-blade bodies, combustion-chamber walls, etc., are made, those plant components subjected to high thermal loading are cooled in a manner known per se by means of cooling-passage systems of different design. Cooling passages, through which, compared with the temperature of the hot gases, relatively cold air is fed, are typically provided in the interior of turbine blades or along the combustion-chamber walls. For example, by the cooling-passage systems arranged downstream of the compressor stages, some of the compressed air is diverted from the air compressor and fed into the cooling passages.
In addition, in order to improve the cooling effect inside the cooling passages, it is known to attach rib features to the inner-wall sides of the cooling passages, which rib features are raised above the inner wall and permit a decisive improvement in the heat exchange between the hot cooling-passage wall and the cooling-air flow. The idea underlying the provision of cooling ribs is to form vortices close to the cooling-passage wall, by means of which vortices the cooling-air mass flow which comes in thermal contact with the cooling-passage inner wall can be increased decisively. Thus so-called secondary vortices form within the cooling-air flow, which is directed axially through the cooling passage, and these secondary vortices have vortex-flow components which are directed perpendicularly to the cooling-passage walls. The forming of such secondary vortices is illustrated in FIG. 2, in which a perspective cross section through a cooling passage 1 known per se is shown. The cooling passage 1 shown in the exemplary embodiment according to FIG. 2 has a square cross section and is therefore surrounded by four equally long cooling-passage walls. In this case, two opposite cooling-passage walls 2, 3 are each provided with rib features 4 arranged one behind the other in the longitudinal direction of the cooling passage. The rib features 4, which are of rectilinear design and have a rectangular cross section, preferably run at an angle to the longitudinal extent of the cooling passage 1 and enclose an angle a of about 45xc2x0 with the longitudinal axis A of the cooling passage. If the cooling-air flow now passes axially through the cooling passage 1, a flow profile which provides two secondary vortices 5, 6 is formed by the rib features 4 in the cross section of flow of the coolant flow. The secondary vortices 5, 6 in turn lead to turbulent intermixing of the boundary layer directly over the cooling-passage inner wall, as a result of which improved cooling-air exchange takes place at the cooling-passage inner wall and a greater heat flow from the hot cooling-passage inner wall to the cooling-air flow is obtained. Based on this knowledge, many studies have been carried out which deal with the effect of the change in parameters determining the rib features on the heat-transfer efficiency, such as changes in the height of the rib features, the spacing of the rib features, the rib orientation relative to the longitudinal axis of the cooling passage, Reynolds and Prandtl number, cooling-passage aspect ratio, etc. However, investigations in this respect were restricted merely to rectilinear rib features.
The object of the invention is to develop an arrangement for cooling a flow-passage wall surrounding a flow passage, having at least one rib feature which induces flow vortices in a flow medium passing through the flow passage, is attached to that side of the flow-passage wall which faces the flow passage, and has a main longitudinal extent which is oriented at an angle of xcex1xe2x89xa00xc2x0 to the direction of flow of the flow medium passing through the flow passage, in such a way that the cooling effect of the arrangement is to be considerably increased without a decisive increase in the production cost compared with conventional measures. The improvements are intended to make it possible to improve the cooling capacity of the cooling-air flow passing through a flow passage, so that a further increase in output is made possible by increased process temperatures inside the gas-turbine plant.
According to the invention, an arrangement according to the preamble of claim 1 is developed in such a way that the rib feature, along the main longitudinal extent, at least partly has rib-feature sections whose axes enclose an angle of xcex2xe2x89xa00xc2x0 with the main longitudinal extent.
The invention proceeds on the basis of the knowledge that rib features preferably running at an angle to the main flow inside a cooling passage generate the secondary vortices which are shown schematically in FIG. 2 and by means of which cool air is transported from the center of the cooling passage to the hot cooling-passage inner walls in order to effectively cool the latter. Unlike the hitherto rectilinear rib elements, the invention provides for the rib elements to be designed so as to be curved about their rib longitudinal axis in such a way that they assume, for example, a serpentine form, which can be constructed in many different ways. An especially preferred embodiment consists in the sinusoidal design of the rib elements, the main orientation of the rib element relative to the main flow being retained as in the known rectilinear rib elements, preferably 45xc2x0 relative to the main flow direction.
A multiplicity of semicircular sections lined up directly next to one another are also suitable for forming geometrical configurations of the rib features according to the invention. For further, possible rib-feature designs, reference is made to the exemplary embodiments and the figures.
Two advantages in particular are associated with the design according to the invention of the rib features, namely a largely unchanged formation of secondary vortices, which leads to active intermixing of the boundary layer close to the inner-wall surface of the cooling passage. Furthermore, a larger surface of the rib features is created by the curved sections provided along the rib features, as a result of which the heat-transfer surface increases. Provided that the heat-transfer coefficient remains largely unchanged compared with the conventional, rectilinear rib elements due to the geometric modification of the rib features, which may be assumed, the heat exchange between the hot cooling-passage inner walls and the cooling air flowing through the cooling passage noticeably increases with the increased heat-transfer surface.